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(Caveat - I don't own any Atlas locos, so my comments refer to a Kato truck, but I think the design is similar.) ......
However, the trucks roll a little bit better with the axle cups in place, so I think the axle cups bear a fraction of the weight
Ok, I did do some 'experimenting' since the last post, and I will walk back my comments somewhat, but not on some key points. I agree that the Atlas designs look similar to the Kato. And I agree it's reasonable to assume that whoever designed those trucks thought they were copying the design principles. However I still maintain that the execution of the Atlas HTC truck fails to do that, and that the truck tower plastic is actually resting on the outside plastic axles at least most of the time. Whether they didn't really understand the Kato principles, or whether they just screwed up the dimensions in the design and tooling process, I can only speculate on. However, the way those outside axle housings are flat on top, and the well documented middle axle 'hop' due to the extra round space and the fact that the cups don't bear down on that axle... those things suggest maybe the designer didn't fully understand what they were trying to achieve. (Not saying I could have done better, just observing the result.)As far as other Atlas trucks, including 2 axle trucks, I'll reserve comment until I have more time to look at a few more. I took apart a GP35 tonight and was frankly surprised at how well the trucks rolled on their own, because in the past I recall very poor rolling examples, particularly with the GE sideframes. With that said, it did seem that the cups might be reliably supporting those trucks on the axles.I was being a bit facetious about Atlas not being low friction, but this is what I mean:That's an HTC truck, parts cleaned and checked and reassembled. And no, the photo is not rotated nor are there any other tricks going on. That board is raised up at about 35 degrees. In my experience this is not a fluke result.The truck does not roll on its own. Period. Not with a little nudge to get it started, like some need. Not at all. You can get the wheels and gears to turn if you apply a minimal downward pressure and move it back and forth. But the truck's own weight does not apply enough grip to the rails to overcome its internal friction. Whether this is the fault of where the weight bears, or the cups, or the gears, I don't know for sure. But it's not a low friction truck!